1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor (for example, a photoconductor drum) with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet through an intermediate transfer belt, followed by heating and pressurization for fixing, whereby an image is formed on the sheet.
The above-described image forming apparatus includes a sheet conveyance section that conveys to an image forming section a sheet fed from a sheet feeding section (sheet feed tray, an external sheet feeding device, and the like). The sheet conveyance section includes a plurality of conveyance roller sections including a loop roller section, a registration roller section and the like, for example. It is known that sheets are conveyed in a tilted state (so-called skew) in the sheet conveyance section due to a slight inclination of a conveyance roller shaft, a difference in a nip pressure (hereinafter referred to as “conveyance nip pressure”) between a plurality of roller pairs in the conveyance roller section, or other reasons. When skew occurs in the case where long sheets are used for an image formation, the sheets successively pass through the conveyance nip in a tilted state, and consequently damages such as paper wrinkle are caused. Examples of techniques for correcting skew of a long sheet include the techniques disclosed in Japanese Patent Application Laid-Open No. 2006-76784 (PTL 1) and Japanese Patent Application Laid-Open No. 2011-235530 (PTL 2).
PTL 1 discloses a technique in which steering rolls which can be displaced along the sheet conveyance direction are disposed on the upstream side and downstream side in an image forming section, and the steering roll are displaced on the basis of deviation of continuous paper detected by a detection member disposed in the vicinity of the steering roll to thereby correct skew of a long sheet.
PTL 2 discloses a technique in which an end guide member configured to guide the ends of a sheet by making contact with the ends of the sheet in the sheet width direction is moved in a direction orthogonal to the sheet conveyance direction to thereby correct skew of a long sheet.
The term “long sheet” as used herein means a sheet having a sheet length greater than that of a sheet (for example, A3-sheet) which can be contained in a commonly used sheet feed tray provided in an image forming apparatus, and the term “long sheet” includes roll paper, continuous paper and the like.
Incidentally, in a fixing device, the nip pressure (hereinafter referred to as “fixing nip pressure”) of a fixing nip is higher than the conveyance nip pressure, and heat is supplied to sheets from a fixing side member (for example fixing roller). Therefore, when a tilted long sheet passes through the fixing nip, damages such as paper wrinkle are easily caused.
According to the techniques disclosed in PTLs 1 and 2, while large skew of a sheet caused in the sheet conveyance path can be corrected, it is difficult to correct small skew that causes problems when the long sheet passes through the fixing nip.